Brosious etal.
pinhole detectors



Nov. 5, 1968 R BROSIOUS ET AL Re. 26,485

PINHOLE DETECTORS 5 Sheets-Sheet 1 Original Filed May 3, 1963 INVENTORSDan/bl R Bros/bus BY James Kfloll/hgshead 1963 D. R. BROSIOUS ET AL26,485

PINHOLE DETECTORS 5 Shee ts-Sheet 2 Original Filed May 3, 1963 N i M m%k SE3 R SE 28 m2 0:

Nov. 5, 1968 0. R. BROSIOUS ET 26,435

PINHOLB DETECTORS Jriginal Filed May 3, 1963 5 Sheets-Sheet 5 INVENTORSDan/Eel R. Bros/bus BY James K. Holllhgshead Nov. 5, 1968 0. R BROSIOUSET AL Re. 26,485

PINHOLE DETECTORS 5 Sheets-Sheet 4 JIiginal Filed May .3, 1963 INVENTORSDaniel A. Bros/bus BY James K Holl/hgshead Nov. 5, 1968 o. R BROSIOUS ETAL Re. 26,485

PINHOLE DETECTORS 'Jriginal Filed May 5, 1963 5 Sheets-Sheet 5 Q u l.

I I: Q

IN VENTORS Dan/e/ R Bros/'00s BY ubmes K hb/I/hgshead United StatesPatent Oflice Re. 26,485 Reissues] Nov. 5, 1968 26,485 PINHOLE DETECTORSDaniel R. Brosious, Bethlehem, and James K. Hollingshead, Coopersburg,Pa., assignors, by mesne assignments, to Bethlehem Steel Corporation, acorporation of Delaware Original No. 3,263,086, dated July 26, 1966,Ser. No.

277,886, May 3, 1963. Application for reissue June 12,

1967, Ser. No. 651,637

5 Claims. (Cl. 250-219) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE An apparatus for detecting pinholes in movingstrip in which light within first and second frequency ranges isdirected toward the strip. A shutter containing a photoelectric cell isautomatically positioned at each edge of the strip in response to lightwithin the second frequency range, light within the first frequencyrange being absorbed by the shutters. A light filter across thedetection chamber passes only light within the first frequency range,such light having been directionally controlled by louvers positionedabove the strip.

Background of the invention This invention relates to apparatus fordetecting imperfections in moving strip by photoelectric means, and moreparticularly to pinhole detectors of novel construction. The inventionfurther relates to improved non-contacting shielding means which can beutilized in said pinhole detectors to improve the detection accuracy ofsaid detectors.

Steel strip, particularly if it is subsequently to be coated withanother metal, e.g. tin, is inspected for pinholes prior to the coatingoperation. The strip, which is accepted or rejected by the customer onthe basis of the number of pinholes detected per area of materialinspected, can thus be used in less critical applications if the densityof pinholes is greater than that which is acceptable to tin-platecustomers.

Inspection is accomplished by means of a pinhole detector, whichcomprises a source of light positioned on one one side of the strip andlight-sensitive means positioned on the other side of the strip.Pinholes are detected through the actuation of the light-sensitive meansby light from the source which has passed through the pinholes in thestrip.

Inasmuch as actuation of the light-sensitive means by light passingaround an edge of the strip would result in false pinhole indications,pinhole detectors are provided with shielding means positioned adjacenteach edge of the strip. The earliest types of shielding means comprisedshutters which were fixed with respect to the detector housing and madeno contact with the edges of the strip. These types of shielding meanswere not very accurate, as slight variations in the width of the stripand lateral movement thereof resulted in false pinhole indications. Inaddition, shutters which were fixed with respect to the detector housingcould not be used in continuous lines inasmuch as the width of the stripin said lines may suddenly change as a coil of strip of one width isfollowed into the detector by a coil of strip of a greater or lesserwidth.

Later types of shielding means comprised shutters of the movable,contacting type, i.e. the shutters were adapted to move in a directiontransverse to the path of the moving strip, and a portion of eachshutter continuously made contact with an edge of the moving strip. Indevices of this type, which were considerably more accurate thanshielding means of the fixed type, the shutter frequently had to bereplaced due to the rapid wear of those surfaces of the shutter whichmade contact with the edge of the moving strip. Furthermore, the speedat which the strip could be inspected was limited to about 1000 feet perminute, inasmuch as at higher speeds vibration of the shutter resultedin loss of contact between the strip and the shutter.

More recently, shielding means have comprised shutters of the movable,non-contacting type. In these devices, each shutter is provided withsensing means provided for detecting the edge of the strip and controlcircuits are continuously actuated by said means to maintain therelative positions of the shutter and the edge of the strip constant.Such shutters, to be effective, have generally been constructed with anarrow throat section and have been maintained in close proximity to theedge of the strip. While constituting a considerable improvement overshutters of the fixed, non-contacting type and the movable, contactingtype, said devices have proved impractical for inspecting strip movingat speeds up to 5000 and 6000 feet per minute. At such speeds wavy-edgedstrip frequently made contact with the shutter, resulting in excessivewear and damage thereto.

The light-sensitive means of the pinhole detector comprises a pluralityof highly sensitive photo-multiplier tubes. Said tubes can detect lightpassing through a hole in the strip of 0.001 inch diameter. For thisreason, all of the aforementioned types of shielding means were designedto block, in the vicinity of the edges of the strip, reflected light andlight emitted from the source in planes both coplanar with andnoncoplanar with planes perpendicular to the surface of the strip andparallel to the direction of strip travel. While each of the foregoingtypes of shielding means was fairly effective at blocking light emittedin planes coplanar with said planes, none was very effective at blockingeither reflected light or light emitted in planes noncopolanar with saidplanes.

Summary of the invention It is an object of this invention to provide apinhole detector constructed so that the probability of light actuatingthe light-sensitive means without first passing through the strip isgreatly reduced.

It is another object of this invention to provide a shielding devicewhich cooperates with said detector to permit accurate inspection ofmoving strip to within a very small distance from its edges. A furtherobject is to provide a shielding device in which no part thereof makesany contact whatsoever with an edge of the strip, and which permitsmoving strip to be accurately inspected at speeds of 5000 feet perminute or more.

A still further object is to provide a shielding device comprising ashutter having an unusually wide and deep throat section whereby theprobability of inadvertent contact between the strip and the shutter isgreatly reduced.

An additional object of this invention is to provide anelectrical-mechanical closed loop servo system in which a shutter iscontinuously re-positioned adjacent an edge of a moving strip inresponse to a continuoussignal from sensing means associated with saidshutter.

We have discovered that the probability of reflected light, as well aslight emitted from the source in planes noncoplanar with planesperpendicular to the surface of the strip and parallel to the directionof strip travel, actuating the light-sensitive means of the detectorwithout first passing through pinholes in the strip can be greatlyreduced by constructing the pinhole detector in the following manner. Asource of .light within a first frequency range is disposed on one sideof the strip and transverse to the direction of strip travel. Mountedbetween said sourceand said strip is a plurality of parallelphoto-luminescent louvers disposed in a plane perpendicular to thesurface of the strip and parallel to the direction of strip travel. Thelouvers absorb all incident light within said first frequency range and,in response to said absorbed light, emit light within a second frequencyrange. Each of said frequency ranges falls within a different portion ofthe frequency spectrum, i.e. the ranges do not-overlap. Thus, thelouvers may he eonsidered to be selective transmission means in 'whichonly light from the source which enters the louvers in planesperpendicular to the surface of the strip and parallel to the directionof strip travel passes through the louvers. Light from the sourcewhich'enters the louvers'in other planes emerges therefrom as lightwithin a second frequency range.

Disposed between the strip and the light-sensitive means in the detectorbase is a substantially monochromatic transmitting filter adapted topass light within said first frequency range. Inasmuch as said filterabsorbs all light except light withinsaid first frequency range and allthe light within said first frequency range in the vicinity of thelight-sensitive means is in a plane perpendicular to the surface of thestrip and parallel to the direction of strip travel, the probability oflight from the source actuating the light-sensitive means without firstpassing through the strip, eg by reflection, is very small.

While the detection accuracy of pinhole detectors of the aboveconstruction,when provided with shielding means adjacent each edge ofthe strip, is superior to that of prior detectors provided with the sametype of shielding means, we have discovered a novel shielding meanswhich utilizes the optical properties of the above-described detector toimprove the effectiveness of shielding at an edge of the strip. Briefly,the shielding means comprises a movable, non-contacting shutter havingan upper shield and a lower shield, each of which is constructed of amaterial which absorbs light within said first frequency range. The topof the upper shield is contiguous to the lower edges of the louvers andthe lower shield rides along the base of the detector. Associated withthe shutter is sensing means connected to appropriate circuits whichactuate electromechanical means to maintain the relative position of theshutter and the edge of the strip constant. Preferably, said sensingmeans is mounted on the lower shield and comprises a photoresistive cellwhich is extremely sensitive to light within said second frequencyrange. ln this case, the shields are constructed of a material which, inaddition to absorbing light within said first frequency range, transmitslight within said second frequency range.

The above described shielding means effectively pre vents light withinsaid first frequency range from passing around an edge of the strip and,in addition, the shutter can be constructed with a wide and deep throatsection, thereby substantially reducing the probability of inadvertentcontact between any part of the shutter and an edge of the strip.

The features of the invention which we believe to be novel are set forthwith particularity in the appended claims. Both the structure and themethod of operation of the invention, as well as further objects andadvantages thereof, will be better understood by reference to thefollowing description taken in connection with the drawings, in whichBrief description of the drawings FIGURE 1 is a perspective view, partlyin section, of the pinhole detector and a portion of the shieldingmeans.

FIGURE 2 is a perspective view, partly in section, of the shieldingmeans.

FIGURE 3 is a sectional view along the lines 33 of FIGURE 2, and FIGURE4 is a sectional view along the a I *4 I lines 44 of FIGURE 3 showingthe details of the drive assembly.

FIGURE 5 is a side elevation view of the shutter and a portion of thepinhole detector.

FIGURE 6 is a plan view of the shutter.

FIGURE 7 is a schematic of a motor control circuit, and FIGURE 8 showsgraphically the wave forms of the voltages at various points in thecircuit.

' Descrjptibnof the preferred embodiments Referringmore. particularly toFIGURE 1, a pinhole detector 10 is shown as comprising a detector head11 and a detector base 12 between which a strip 13 travels. The detectorhead contains a source of light within a first frequency'range; forexample fluorescent tubes 14 which emit substantially monochromaticultraviolet light. Mounted directly below the source is a plurality ofparallel photoluminescent louvers 15 disposed in a plane perpendicularto the surface of the strip and parallel to the direction of striptravel. The louvers are journaled on a pair of rods 16 which extend thelength of the detector head. Adjacent louvers are separated by means ofpairs of square spacers 17 also journaled on said rods.

The louvers and spacers are rendered photoluminescent by covering thesurfaces thereof with either a paint or a tape having photoluminescentproperties. Each louver and spacer, by reason of its photoluminescentproperties, absorbs all incident ultraviolet light and, in response tosaid absorbed light, emits lightwithin a second frequency range, e.g.light in the near infrared region. A tape having theabove-describedproperties, sold under the trademark Scotchcal, andidentified as #3483 Warning Orange, may be used. Thus, only ultravioletlight from the tubes 14 which enters the plurality of louvers 15 inplanes perpendicular to the surface of the strip 13 and parallel to thedirection of strip travel passes through the louvers. Incidentultraviolet light, i.e. ultraviolet light from the tubes 14 which entersthe plurality of louvers 15 in other planes and thus impinges upon theindividual louvers and spacers, emerges therefrom as near infraredlight.

As a specific example of the above, we have provided a detector headwith one hundred and sixty-eight louvers measuring one inch high by fourand one-half inches long by .006 inch thick. Said louvers are separatedby threequarter inch square spacers, one-quarter inch thick. Allultraviolet light leaving the source at an angle greater than fifteendegrees with respect to a plane perpendicular to the surface of thestrip and parallel to the direction of strip travel is absorbed by thephotoluminescent surfaces of the louvers and the spacers. In response tosaid absorbed light, light in the near infrared region is emitted.

Ultraviolet and near infrared light leaves the detector head 11 throughwindow 17a. Said window, which is disposed between the head 11 and thestrip 13, prevents dirt, dust, etc. from entering the detector head 11.

The detector base 12 comprises a detection chamber 18 in whichlight-sensitive means, e.g. a plurality of photomultiplier tubes 19, ishoused. Reflecting elements, not shown, are provided for directingincoming light toward said photomultiplier tubes. The detection chamberis pro vided with a slotted aperture 20 in the upper section thereof.The aperture is of sufficient length to accommodate the greatest widthof material it is desired to in spect, and is aligned directly belowfluorescent tubes 14.

The chamber is impervious to all light except that which enters throughsaid aperture.

Mounted directly above the aperture 20 is a substantially monochromaticultraviolet transmitting filter 21 through which all light entering thedetection chamber 18 must pass. The filter is adapted to pass only lightwithin the same frequency range as that emitted by the light sourcehoused in the detector head 11.

The detector is provided with means for preventing light from the sourcefrom passing around the edges of the strip. While any of theaforementioned types of shielding means may be employed for thispurpose, in the preferred embodiment of the invention shielding meanscomprising a moveable, non-contacting shutter is utilized. Such ashielding means is shown broadly in FIGURE 1 as comprising a shutter 22comprising an upper shield 23 and a lower shield 24 mounted on a tailplate 25. The shutter is positioned adjacent an edge of the strip 13 andis adapted to be driven in a path transverse to the direction of striptravel. A pair of rub rails 26 is provided to guide the shutter alongthe detector base. A housing 27 mounted on an extension plate 28extending longitudinally of the detector base houses the drive assemblyfor one of the shutters.

Pinhole detectors of the above construction have proved to be extremelyeffective in preventing light which has not passed through the stripfrom entering the detection chamber and actuating the photomultipliertubes housed therein. Inasmuch as the filter 21 absorbs all light exceptultraviolet and substantially all the ultraviolet light in the vicinityof the strip above the detection chamber is in a plane perpendicular tothe surface of the strip and parallel to the direction of strip travel,the probability of the photomultiplier tubes being actuated by lightwhich has not first passed through the strip, e.g. by reflection, isvery small.

The improved shielding means will next be described in detail. Inasmuchas the mechanical construction of said means is substantially identicalfor each of the two shielding means necessary for the inspection ofmoving strip, the description will be limited to means positionedadjacent only one edge of the strip.

As is shown in FIGURE 2, the improved shielding means broadly comprisesa shutter 22 comprising an upper shield 23 and a lower shield 24. Theshields are constructed of a material which absorbs light in theultraviolet region and transmits light in the near infrared region. Thelower shield is mounted on a tail plate 25 which is slidably mounted inhousing 29.

The end of the shutter adjacent the strip is provided with sensing meansfor producing an electrical signal indicative of the relative positionof the shutter with respect to the edge of the strip. In the preferredembodiment of the invention, said sensing means comprises aphotoresistive cell 30 securely mounted in the lower shield. Other typesof sensing means, e.g. electromagnetic or electrostatic means, may, ofcourse, also be used.

The resistance of the photoresistive cell 30, which is characterized byextreme sensitivity to light in the near infrared region, variesinversely with the intensity of the near infrared light which impingesupon it. The near infrared light, the source of which is the louvers l5and the spacers 17, passes substantially unimpeded through the upper andlower shields. While the near infrared is available to actuate thephotoresistive cell, it cannot enter the detection chamber 18 becausethe ultraviolet transmitting filter 21 covering the aperture 20 absorbslight in the near infrared region.

The photoresistive cell 30 is electrically connected to a motor controlcircuit, designated in FIGURE 2 as the square 31. The motor controlcircuit is in turn electrically connected to reversible motive powermeans, e.g. D-C torque motor 32, which is adapted to drive the shutter22 transverse to the direction of strip travel.

To operate the device, the shutter 22 is initially driven inwardly bythe DC motor 32 until a portion of the photoresistive cell 30 isdirectly beneath the edge of the strip 13. This inward motion is causedby the relatively high intensity of near infrared light initiallyimpinging upon the cell which results in a relatively large outputcurrent to the motor control circuit 31. The motor control circuit, thedetailed operation of which will be described in full detail later inthe specification, causes the motor 32 to drive the shutter inwardlywhen the light impinging upon the photoresistive cell is of relativelyhigh intensity. The inward motion of the shutter ceases when the outputcurrent of the photoresistive cell decreases to a certain predeterminedequilibrium value, which occurs when the edge of the strip prevents thenear infrared light emitted from the louvers from impinging upon theentire photosensitive surface of the cell, ie when the cell is onlypartially illuminated. The cell is so positioned in the shutter that noportion of the shielding device makes any contact whatsoever with theedge of the strip.

When the output current of the photoresistive cell 30 is at itsequilibrium value, the motor control circuit 31 causes the D-C motor 32to develop no net torque, and the position of the shutter 22 is thusfixed with respect to the position of the edge of the strip 13. As therelative positions of the edge of the strip and the shutter, andtherefore the photoresistive cell, vary due to lateral motion of thestrip and variations in the width thereof, the output current of thephotoresistive cell varies correspondingly. The motor control circuit,in response to deviations from the equilibrium value of photoresistivecell output current, causes the reversible D-C motor to develop a nettorque in the direction which re-positions the shutter with respect tothe strip so that the output current of the photoresistive cell is againat the equilibrium level.

As shown in FIGS. 5 and 6, the shutter 22 broadly comprises a tail plate25 which provides a base upon which the remaining components of theshutter are mounted. Rigidly mounted on the bed of the detector base 12are rub rails 26 having grooves 33 which serve to guide the shutter andprevent lateral motion thereof. The forward portions of the tail plateand the lower shield are provided with a longitudinal cut 34 so that thestrip may be accurately inspected to within a small distance from itsedges.

Rigidly mounted on the front portion of the tail plate is the lowershield 24. The upper surface of the lower shield is below the top of rubrails 26, thereby protecting said shield from the strip. The uppershield is connected to the lower shield by supporting member 35, whichmay, as shown, be integral with the upper shield. Mounted on each sideof the supporting member is a light shield 36 which aids in preventingreflected light, as well as ambient light, from entering the detectionchamber 18.

The upper and lower shields 23 and 24, supporting member 35, and lightshields 36 are preferably constructed of transparent red acrylic plasticwhich passes light in the near infrared and absorbs light in theultraviolet region. All surfaces thereof are highly polished to permitmaximum ultraviolet light absorption and near infrared lighttransmission.

The above-described shields and supporting member effectively preventultraviolet light from passing around an edge of the strip. In addition,the throat formed by said components is relatively wide and deep, thusgreatly reducing the probability of damage to the shutter by contactthereof with the moving strip.

The photoresistive cell 30 is mounted in the front portion of the lowershield by providing therein a cylindrical housing 37 into which the cellis inserted. The cell is secured by providing a pressure adjustmentscrew 38 which extends inwardly from the side of the lower shield.

Mounted on the rear portion of the tail plate 25 and extendinglengthwise along the center thereof is a T-bar 39 comprising a base 40and a vertical section 41. The rear of the vettical section is curved toprovide a mechanical stop. A button head cap screw 42 is mounted on thebase 40 and serves as a cam to actuate limit switch 43, which dependsfrom the roof of housing 29. Rigidly mounted at the rear of the tailplate is an electrical connection base 44 to which the wires from thephotoresistive cell are connected.

The shutter housing 29 broadly comprises side plates 45 and 46, cover47, bottom plate 48, and motor mounting bracket 49. The housing 29 ismounted on the detector base extension 50. The motor mounting bracket 49is rigidly attached to the top surfaces of side plates and 46, and hasits forward section 51 bent downwardly along the front walls of saidside plates. Forward section 51 is provided with a slot 52 through whichthe vertical section 41 of T-bar 39 passes during inward or outwardmovement of shutter 22.

Mounted on the motor mounting bracket is the motor housing 53, whichprovides a base for the motor 32. The shaft 54 and drive wheel 55 of themotor pass through a circular opening 56 disposed in the bottom of themotor housing and extending through the motor mounting bracket 49 intothe shutter housing. Mounted on one wall of the motor housing is aconduit 57 through which the electrical conductors connecting the motorand the photoresistive cell to the motor control circuit 31 are passed.A cover for the motor housing may of course be provided.

A hinge plate assembly, shown in detail in FIGURES 3 and 4, is disposedbelow the motor mounting bracket and comprises a hinge plate 58 to whichare pinned guide rollers 59 and 60 by pins 61 and 62, respectively. Thehinge plate is rotatably mounted on spring bar 63 by pin 64, and thespring bar is rotatably mounted on the spring bar stud 65. The upperportion of the shank of the spring bar stud is threaded and passesthrough the motor housing 53 and motor mounting bracket 49 and is boltedthereto. The spring 66 is attached to the spring bar 63 by screw 67 anda cam wheel 68 having an olfcenter shaft 69 is disposed so that it makescircumferential contact with the free end of the spring. The shaft 69 ofthe cam wheel passes through the motor mounting bracket and is mountedon the motor housing. A slotted surface 70 is provided at the top of theshaft so that it can be rotated.

The hinge plate assembly operates in combination with the drive wheel 55of motor 32 to drive the shutter 22 inwardly and outwardly. The verticalsection 41 of T-bar 39 is engaged in frictional contact between drivewheel 55 and guide rollers 59 and 60. The pressure exerted by the drivewheel and the guide rollers against section 41 may be varied by rotatingthe shaft 69 of cam wheel 68.

One end of a flexible connector 71 is connected to connection base 44.Connector 71, which conducts current to and from the photoresistivecell, has its other end connected to a connection base (not shown)rigidly mounted on the lower side of motor mounting bracket 49. Suitableconductors connected thereto pass through conduit 57 to motor controlcircuit 31.

The motor control circuit 31 broadly comprises the four followingfunctional circuits: (1) an output power stage; (2) driver or amplifierstages; (3) a height to width converter; (4) a pulse generator. Theoutput power stage comprises power transistors 101 and 102 and ajunction diode 103. Transistor 101 is of the p-n-p type and has anemitter electrode 104, a collector electrode 105, and a base electrode106. Transistor 102 is also of the p-n-p type and has an emitterelectrode 107, a collector electrode 108, and a base electrode 109.Transistors 101 and 102 function like a pair of interlocked single poleswitches connected so that when one switch is open, the other switch isclosed. That is, when transistor 101 is in saturated conduction,transistor 102 is cut off, and vice versa. A conductor connects thejunction 111 of diode 103 and the emitter 107 of transistor 102 toconductor 112 and, assuming limit switch 43 is closed, to one side ofthe motor 32, the other side of the motor being connected to ground.Connected in parallel with limit switch 43 is a resistor 164 and ajunction diode 165. Connected to the emitter 104 of transistor 101 is asource 113 of positive DC potential, cg. +24 volts. Connected throughload resistor 114 to the collector 108 of transistor 102 till is asource 115 of negative D-C potential equal in magnitude to that ofsource 113, e.g. 24 volts.

If it is assumed that transistor 101 is initially in saturatedconduction, current flows from positive source 113 through transistor101. The collector courrent leaves the collector 105 and fiowssubstantialy unimpeded through diode 103, which is forward biased to thecollector current from transistor 101, to junction 111. The base 109 oftransistor 102 is connected to the positive side of diode 103 byconductor 116 and the emitter 107 is connected to the negative sidethereof, so that the small voltage drop across the diode 103 when thesaturated collector current of transistor 101 flows therethroughsuffices to reverse bias transistor 102 beyond cut-off. i

If transistor 101 is cut off, only leakage current flows in the basecircuit thereof, and the collector currentis quite small. The voltagedrop across diode 103 is therefore negligible, and transistor 102 is nolonger reverse biased. Sufficient current is suplied to the base 109through resistor 117 to drive transistor 102 into saturation.

The operation of the output power stage is predicated upon the lowforward impedance of junction diodes and the switching properties oftransistors, as is well known in the art. That is, both a forward biasedjunction diode and a transistor in saturated conduction may beconsidered substantially short circuits. Thus, when transistor 101 is insaturated conduction and transistor 102 is cut off, current flowssubstantially unimpeded from positive source 113 through transistor 101and diode 103 directly into motor 32, and the positive D-C voltage atsource 113 is, for all practical purposes, directly across said motor.Similarly, when transistor 101 is cut oil and transistor 102 is insaturated condition, current flows substantially unimpeded from motor 32through transistor 102 into negative source 115, and the negative D-Cvoltage at source 115 is, for all practical purposes, directly acrosssaid motor. Since transistors 101 and 102 are interlocked, power iscontinuously supplied to motor 32.

Transistors 101 and 102 are alternately driven into saturation at apredetermined frequency, e.g. 500 cycles per second. If each transistoris driven into saturated conduction for 50% of each cycle, the positiveand negative D-C voltage sources 113 and 115 are alternately placedacross the motor 32 for equal periods of time by the switching action ofsaid transistors. Provided the frequency of switching is sufficientlyhigh, the shaft 54 of motor 32 will not rotate, since the torquedeveloped is of insufilcient magnitude to overcome the mechanicalinertia of the motor and its load, The equilibrium condition, i.e. thecondition in which transistors 101 and 102 are driven into saturatedconduction for 50% of each cycle, occurs when the shutter 22 is properlypositioned with respect to the edge of the moving strip as determined bythe intensity of the near infrared light impinging upon thephotoresistive cell 30. The voltage waveform across the motor atequilibrium is shown graphically in FIGURE 8(A). Should the shutter 22be momentarily improperly positioned, near infrared light of a greateror lesser intensity will impinge upon the photoresistive cell 30, andtransistors 101 and 102 will no longer be in saturated conduction for50% of each cycle. FIGURES 8(8) and 8(C) show the voltage waveformacross the motor 32 during such non-equilibrium conditions. During nonequilibrium conditions, one of the D-C voltage sources 113 and 115 isacross the motor for a majority of each cycle, and a net torque isdeveloped by the motor 32 which causes the shaft 54 to rotate in such adirection that shutter 22 is re-positioned in its equilibrium position;The direction and magnitude of the torque developed depends upon theintensity of the near infrared light impinging upon the photoresistivecell 30.

The driver or amplifier stages comprise p-n-p transistor 118 and n-p-ntransistor 119 and serve to reverse bias transistor 101 in response toan input control signal de pendent upon the output current from cell 30.Transistor 118 has an emitter electrode 120, a collector electrode 121,and a base electrode 122. Transistor 119 has an emit ter electrode 123,a collector electrode 124, and a base electrode 125. Transistors 119,118, and 101 are directly coupled to each other so that when transistor119 is in saturated conduction, transistors 118 and 101 are in saturatedconduction, also. Similarly, when transistor 119 is cut off, transistors118 and 101 are cut off, also. In other words, the state of conductionof transistor 119 determines the state of conduction of transistors 118,101, and because transistors 101 and 102 are interlocked, 102.

The base 125 of transistor 119 is connected to junction 126. In theabsence of a negative potential at junction 126, transistor 119 is insaturated conduction due to the current supplied to its base 125 throughresistor 127. Current then flows from source 113 through resistor 128and through transistor 119 to ground. The base 122 of transistor 118 isconnected to resistor 128 at junction 129. The voltage drop acrossresistor 128 when transistor 119 is in saturated conduction cause thepotential at junction 129 to become less positive, and thereby providesforward bias to transistor 118. The collector 124 of transistor 119supplies sufficient base current to the base 122 of transistor 118 todrive it into saturated conduction. Connected between the emitter 120 oftransistor 118 and the base 106 of transistor 101 is a current limitingresistor 130, which limits the emitter current of the transistor 118 toa safe value. At the junction 131 of resistor 130 and base 106, aresistor 132 is connected to source 113. Current from source 113 isconducted through resistors 132 and 130 through transistor 118 toground. The voltage drop across resistor 132 when transistor 118 is insaturated conduction causes the potential at junction 131 to become lesspositive, and thereby provides sufiicient forward bias to transistor 101to drive it into saturated condition. As previously explained,transistor 102 is cut oil when transistor 101 is in saturatedconduction.

A negative potential at junction 126 sufiices to reverse bias transistor119 beyond cut off. The potential at junction 129 then becomes morepositive, inasmuch as there is a very small voltage drop across resistor128 when transistor 119 is cut off. This increase in potential atjunction 129 cuts olf transistor 118. The potential at junction 131 thenbecomes more positive, inasmuch as the voltage drop across resistor 132is very small when transistor 118 is cut off. The increase in potentialat junction 131 cuts off transistor 101, thereby permitting transistor102 to be driven into standard conduction.

It can be seen from the foregoing that the presence of a negativepotential at junction 126 for 50% of each cycle causes the sources ofpositive and negative potential 113 and 115, respectively, to bealternately placed across the armature of motor 32 for equal periods oftime. Such a condition prevails when the shutter 22 is properlypositioned with respect to the edge of the strip 13, and no torque isdeveloped by the motor 32. Furthermore, it can be seen that by varyingthe percentage of each cycle during which a negative potential ispresent at junction 126, either the positive source 113 or the negativesource 115 will be placed across the motor 32 for more than 50% of eachcycle. This condition prevails when the shutter 22 is improperlypositioned with respect to the edge of the strip 13, and a net torque isdeveloped by the motor 32 which repositions the shutter 22 in itsequilibrium position.

The output current from the photoresistive cell 30, in response to theintensity of the near infrared light impinging upon it, determines thepercentage of each cycle during which a negative potential is present atjunction 126 by means of circuits now to be described. An n-p-ntransistor 133 has an emitter electrode 134, a collector electrode 135,and a base electrode 136. The emitter 134 is connected to junction 137,and a capacitor 138 is connected between junctions 137 and 126. A loadresistor 139 is connected between junction 137 and ground. Connected tothe base 136 at a junction 140 is a leak resistor 141. Thephotoresistive cell 30 is connected to junction 140 by conductor 142.Connected between junctions 137 and 140 is a filter capacitor 139a.

An n-p-n transistor 143 having an emitter electrode 144, a collectorelectrode 145, and a base electrode 146 has its collector connected byconductor 147 to a junction 148 which is connected to collector 135 oftransistor 133 by conductor 149. A resistor 150 is connected betweenjunction 148 and a source 151 of regulated positive D-C potential, e.g.+24 volts. The emitter 144 of transistor 143 is connected to ground byconductor 152. Connected to the base 146 of transistor 143 at junction153 is a resistor 154 which connects the base 146 to source 151.

A unijunction transistor 155 having an emitter electrode 156 and firstand second base electrodes 157 and 158, respectively, has its base 157connected to ground and its base 158 connected to source 151 throughresistor 159. At a junction 160 is a potentiometer 161 connected inseries with a resistor 161a, whereby the emitter 156 is connected tosource 151. Connected between junctions 160 and 153 is a capacitor 162.

The operation of the circuits is substantially as follows. Transistor155 operates in conjunction with potentiometer 161, resistor 161a, andcapacitor 162 to produce a sawtooth wave voltage, shown in FIGURE 8(D),at junction 160. As is well known in the art, the interbase resistanceof transistor 155 is dependent upon its emitter current. This resistanceis very high until the voltage between the emitter 156 and the base 158reaches a firing potential, which is approximately two-thirds thevoltage between base 157 and base 158, at which time the emitter currentflows, and the resistance of base 157 rapidly decreases to a low value.As the resistance of base 157 decreases, the voltage at the emitter 156also decreases until it is below a minimum or cut off value, at whichtime base 157 returns to its high resistance state and the emittercurrent ceases to flow.

If it is assumed that the power is just turned on, negligible currentflows through transistor 155, since the voltage at its emitter 156 iszero. The emitter voltage increases as capacitor 162 charges throughpotentiometer 161, resistor 161a, and the base-emitter circuit oftransistor 143. During this portion of the cycle, i.e. the time duringwhich the voltage at junction 160 is increasing, the charging currentthrough the base-emitter circuit of transistor 143 provides sutficientforward bias to maintain transistor 143 [is] in saturated conduction.Junction 153 is essentially at ground potential, since the voltage dropfrom the base 146 to the emitter 144 is negligible during saturation.When the voltage at the emitter 156 reaches the firing potential, theresistance of base 157 rapidly decreases to a low value, and thepositive charge stored in capacitor 162 rapidly discharges through theemitter 156 and base 157 to ground until the emitter voltage decreasesto below the cut oil value.

Inasmuch as the voltage across a capacitor cannot change instantaneouslyand the voltage at junction 160 has decreased almost instantaneously,the voltage at junc tion 153 must instantaneously decrease an equalamount. Because the voltage at junction 153 was approximately at groundpotential during the charging cycle of capacitor 162, it must decreasebelow ground potential at the start of the discharging cycle ofcapacitor 162. The negative voltage at junction 153 cuts off transistor143 until said voltage can discharge, the discharge time being primarilya function of capacitor 162 and resistor 154. The value of resistor 154should be such that transistor 143 is cut off for about 10% of eachcycle, although other values may, of course, be [premissible]permissible. The waveform of the voltage at junction 153 is shown inFIGURE 8(E).

As previously pointed out, when the transistor 143 is in saturatedcondition the voltage drop across it is negligible. Since transistor 133is in series with resistor 139 and the combination is in parallel withtransistor 143, transistor 133 and resistor 139 are shorted out duringthat period of each cycle during which transistor 143 is in saturatedconduction. When transistor 143 is cut off, the voltage at junction 148is nearly equal to that of the source 151, and current may flow throughtransistor 133. The waveform of the voltage at junction 148 is shown inFIGURE 8(F).

The photoresistive cell 30 is connected to an adjust able voltagesupply, broadly designated as 163, which is adjusted to cause thephotoresistive cell to supply adequate forward bias through conductor142 to the base 136 of transistor 133 when the cell is fullyilluminated. When the system is operating, the base bias varies from afew microamperes to about four hundred microamperes at fullillumination. The magnitude of the current through transistor 133 ismodulated by the magnitude of the base bias. From the foregoingdiscussion, it can be s en that the voltage at junction 137 is a seriesof narrow pulses, each having an amplitude dependent upon the intensityof the near infrared light impinging upon the photoresistive cell 30.The waveform of this voltage is shown in FIGURES 8(0), (H), and (I) forthree different intensities of light. FIGURE 8(0) shows the voltage whenthe light intensity is relatively large. FIGURE 801) shows the voltageat the equilibrium light intensity, and FIGURE 8(1) shows the voltagewhen the light intensity is relatively small.

Transistor 119, which was previously considered in he driver stages,operates in conjunction with capacitor 138 and transistor 133 to convertthe pulses of varying amplitude and constant width at junction 137 topulses of varying width and constant amplitude at junction 131. When apulse arrives at junction 137 the capacitor 138 rapidly charges throughresistor 150, transistor 133, and the baseemitter circuit of transistor119. Inasmuch as transistor 119 is in saturated conduction by reason ofthe forward bias supplied to its base 125 through resistor 127, junction126 remains at approximately ground potential. When the trailing edge ofthe pulse arrives, i.e. when transistor 133 is cut 011 by theshort-circuiting action of transistor 143, the current through resistor139 decreases instantaneously to a value determined by the impedance ofthe discharging circuit of capacitor 138, i.e. the series impedance ofresistor 127, capacitor 138, and resistor 139. The resistance ofresistor 127 is large compared to that of resistor 139, and the voltageat junction 137 therefore decreases instantaneously to a value slightlyabove ground potential. Inasmuch as the total voltage across capacitor138 cannot change instantaneously, the voltage at junction 126, whichwas previously slightly above ground potential, must instantaneouslydecrease by an amount equal to the decrease in voltage at junction 137.The voltage at junction 126 thus becomes negative until capacitor 138can discharge through resistors 127 and 139.

The negative voltage at junction 126 suffices to reverse bias transistor119 beyond cut off. The percentage of each cycle during which transistor119 is cut off depends upon the magnitude of the voltage at junction126, which in turn depends upon the magnitude of the voltage at junction137. The latter voltage depends upon th emitter current of transistor133, which is dependent upon the intensity of the near infrared lightimpinging upon the photoresistive cell 30. The values of resistor 127and capacitor 138 are chosen so that when the cell 30 is fullyilluminated the RC time constant is long enough to keep transistor 119reversed biased beyond cut-ofl? until another pulse arrives at theemitter 134 of transistor 133. With less than maximum illumination ofcell 30, transistor 119 is cut off for a proportionately shorter periodof time. For example, when the illumination is such that the equilibriumvalue of output current flows from the cell, i.e. when the shutter 22 isproperly positioned alt 12 with respect to the edge of the strip 13,transistor 119 is reverse biased beyond cut-off for 50% of each cycle.

The preferred embodiment of the invention operates in substantially thefollowing manner. The motor 32 is connected so that when a positivepotential is placed across its armature due to the conduction oftransistor 101, the shaft 54 tends to rotate in a direction which wouldcause the shutter 22 to be driven inwardly, i.e. toaway from the strip.When a negative potential is placed across its armature due to theconduction of transistor 102, the shaft 54 tends to rotate in adirection which would cause the shutter 22 to be drawn inwardly, i.e.toward the strip.

The photoresistive cell 30 is provided with power by an adjustable powersupply 163. Between the cell and power supply 163 is a relay 166 whichmay be manually opened to de-energize the cell.

Prior to operating the device, voltage supply 163 is adjusted so thatthere is no output current from the photoresistive cell 30. The systemis now energized, and the shutter is immediately driven to its fullyretracted position inasmuch as there is no input signal to preventtransistor 101 from conducting, and thereby placing a positive potentialacross the armature of motor 32, for the entire cycle. In its fullyretracted position, the head of screw 42 mounted on the base 40 of T-bar39 contacts and opens limit switch 43, throwing the parallel combinationof junction diode 165 and resistor 164 into the circuit. The highimpedance of the diode 165, which is reverse biased, and resistor 164decreases the armature current to a value of insuflicient magnitude tocause further rotation of the shaft 54 of motor 32.

The voltage supply 163 is now adjusted to increase the current throughthe cell 30. The increasing current results in the reverse biasing oftransistor 101 for a portion of each cycle. During that portion of thecycle in which transistor 101 is reverse biased a negative potential isplaced across the armature of motor 32. Inasmuch as diode 165 is thenforward biased, the armature current is relatively unimpeded, andsufiicient torque is developed by motor 32 to drive the shutter 22inwardly and thereby close limit switch 43. The voltage supply 163 isfurther adjusted until the cell 30 is supplying sufficient current toadvance the shutter 22 to the edge of the strip 13 without anyoscillatory motion of the shutter.

After voltage supply 163 has been adjusted, relay 166 is opened and theshutter 22 is driven to its fully retracted position. To begininspection of the strip 13, relay 166 is closed, thereby closing thephotoresistive cell circuit. Inasmuch as the cell 30 is fully exposed tothe near infrared light emitted from the louvers 15 and spacers 17, alarge output current flows from cell 30 to the base 136 of transistor133. The current impulses at junction 137 are therefore very large, andthe voltage drop at junction 126 when the trailing edge of the pulsearrives at junction 137 is sufficient to cut off transistor 101 for theentire cycle. Transistor 102 is therefore in saturated conduction forthe entire cycle, and the negative potential of source 115 is across thearmature of motor 32 for the entire cycle. The shaft 54 of motor 32develops a high torque which drives the shutter 22 inwardly until aportion of the cell 30 is no longer illuminated by near infrared lightfrom the louvers 15 and the spacers 17. Movement of the shutter stopswhen the output current from cell 30 is just sufiicient to reverse biastransistor 101 beyond cutoff for 50% of each cycle.

As the intensity of the near infrared light impinging upon thephotoresistive cell 30 varies due to lateral motion of the strip 13 andvariations in the width thereof, the output current from cell 30 variescorrespondingly and, by means of the motor control circuit 31, causesmotor 32 to re-position shutter 22 accurately with respect to the edgeof the strip. At no time does any portion of the shutter make contactwith the edge of the strip.

The aforementioned motor control circuit has several safety featuresincorporated therein, so that the most common malfunctions thereof willnot result in damage to the strip. For example, since the most commonfailure of transistors and diodes is shorting within the semiconductormaterial, the circuit has been designed so that the shutter will beimmediately retracted should shorting occur in any one of transistors101, 118, 119, 133, 143, 155, or diode 103. In addition, any opencircuit in the photoresistive cell circuit or failure of the lightsource causes the shutter to be immediately retracted.

The following components may be utilized in the circuit shown in FIG. 7.

Resistor 114 ohms. Resistor 117 125 ohms. Resistor 127 100K ohms.Resistor 128 4.7K ohms. Resistor 130 270 ohms. Resistor 132 ohms.Resistor 139 10K ohms. Resistor 141 150K ohms. Resistor 150 2.2K ohms.Resistor 154 4.7K ohms. Resistor 159 330 ohms. Potentiometer 161 K ohms.Resistor 161a 25K ohms. Resistor 164 40 ohms. Capacitor 138 .1 mid.Capacitor 139a .01 mfd. Capacitor 162 .05 mfd. Diodes 103, 165 z Type40-H. Transistors 101, 102 Type 2N278-DS50l-2N1l00. Transistor 118 Type2N1125. Transistor 119 Type 2Nl0l2. Transistor 133 Type 2N78-2N35.Transistor 143 Type 2N356-A. Transistor 155 Type 2N491.

It is to be understood that the values for the circuit components mayvary according to the design for any particular application. Theforegoing specifications were given for the purpose of example only, andare suitable for operations in which the control circuit has arepetition rate of 500 pulses per second.

The aforementioned motive power means utilized to drive the shuttershould preferably be a D-C torque motor. Because of the high outputtorque to inertia ratio that is possible, and also because of the smallinput power required to obtain a given torque, such a motor is ideallysuited for use in the instant device. A further advantage lies in thevery fast torque vs. time response inherent in D-C torque motors.

Changes and modifications of this invention will undoubtedly occur tothose skilled in the art and we therefore do not wish to be limited tothe exact embodiments shown and described but may use suchsubstitutions, modifications or equivalents thereof as are embracedwithin the scope of our invention or as pointed out in the claims,

We claim:

1. In apparatus for detecting imperfections in moving strip byphotoelectric means,

(a) a source of light within a first frequency range positioned on oneside of said strip,

(b) first light-senstive means positioned on the other side of saidstrip,

(c) selected transmission means disposed between said source and saidstrip for converting to light within a second frequency range lightwithin said first frequency range emitted from said source toward saidstrip in planes noncoplanar with planes sub stantially perpendicular tothe surface of the strip and parallel to the direction of strip travel,

(d) a shutter positioned adjacent each edge of the strip and adapted tobe driven transverse to the direction of strip travel,

(c) said shutter comprising a first shield and a second shield,

(f) said first shield extending inwardly of the edge of the stripbetween said strip and said first lightsensitive means,

(g) said second shield extending inwardly of the edge of the stripbetween said strip and said selective transmission means,

(h) said shields being characterized by the property of absorbing lightwithin said first frequency range, said second shield further beingcharacterized by the property of transmitting light within said secondfrequency range.

(i) second light-sensitive means sensitive to light Within said secondfrequency range and positioned in said first shield to receive lightwhich passes around the edge of the strip,

(j) motive power means for driving said shutter,

(k) control means for energizing said motive power means in response tothe intensity of light impinging upon said second light-sensitive means,and

(1) means disposed between said strip and said first light-sensitivemeans for transmitting only light within said first frequency range.

2. In apparatus for detecting imperfections in moving strip byphotoelectric means,

(a) a source of substantially monochromatic ultraviolet light positionedon one side of said strip,

(b) light-sensitive means positioned on the other side of said strip,

(c) a plurality of substantially equidistantly spaced louvers disposedbetween said source and said strip,

(d) said louvers being characterized by the property of absorbingincident ultraviolet light and, in response to said absorbed light,emitting light in the near infrared region,

(e) each of said louvers being disposed in a plane substantiallyperpendicular to the surface of the strip and parallel to the directionof strip travel,

(f) a shutter positioned adjacent each edge of the strip and adapted tobe driven transverse to the direction of strip travel,

(g) said shutter comprising a first shield and a second shield,

(h) said first shield extending inwardly of the edge of the stripbetween said strip and said light-sensitive means,

(i) said second shield extending inwardly of the edge of the stripbetween said strip and said louvers,

(j) said shields being characterized by the property of absorbingultraviolet light, said second shield further being characterized by theproperty of transmitting near infrared light,

(k) a photoresistive cell sensitive to near infrared light andpositioned in said first shield to receive light which passes around theedge of the strip,

(1) motive power means for driving said shutter,

(In) control means for energizing said motive power means in response tothe intensity of the light impinging upon said photoresistive cell, and

(11) means disposed between said strip and said lightsensitive means fortransmitting only ultraviolet light.

3. An apparatus as recited in claim 1, in which said selectivetransmission means comprises a plurality of substantially equidistantlyspaced louvers characterized by the property of absorbing incident lightwithin said first frequency range and, in response to said absorbedlight, emitting light within said second frequency range.

4. In apparatus for detecting imperfections in moving strip byphotoelectric means,

(a) means, disposed on one side of said strip, adapted to emit lightwithin a first and a second frequency range; said means further beingadapted so that light within said first frequency range impinges uponsaid strip in planes substantially perpendicular to the surface of saidstrip and parallel to the direction of travel thereof,

(b) first light-sensitive means positioned on the other side of saidstrip,

(c) a shutter positioned adjacent each edge of the strip and adapted tobe driven transverse to the direction of strip travel,

(d) said shutter comprising a first shield and a second shield,

(e) said first shield extending inwardly of the edge of the stripbetween said strip and said first light-sensitive means,

(f) said second shield extending inwardly of the edge of the stripbetween said strip and means (a),

(g) said shields being characterized by the property of absorbing lightwithin said first frequency range, said second shield further beingcharacterized by the property of transmitting light within said secondfrequency range,

(/1) second light-sensitive means sensitive to light within said secondfrequency range and positioned in said first shield to receive lightwhich passes around the edge of the strip,

(i) motive power means for driving said shutter,

(j) control means for energizing said motive power means in response tothe intensity of light impinging upon said second light-sensitive means,and

(k) means disposed between said strip and said first light-sensitivemeans for transmitting only light within said first frequency range.

5. In apparatus for detecting imperfections in moving strip byphotoelectric means,

said apparatus comprising:

(i) means, disposed on one side of said strip, adapted to emit lightwithin a first and a second frequency range; and

(ii) means, disposed on the other side of said strip, adapted totransmit only light within said first frequency range into the detectionchamber of said apparatus;

an improved shutter comprising:

(a) a first shield disposed between means (ii) and said strip andextending inwardly of an edge of said strip;

(b) a second shield disposed between means (i) and said strip andextending inwardly of said edge of said strip;

(c) said shields being characterized by the property of absorbing lightwithin said first frequency range, said second shield further beingcharacterized by the property of transmitting light within said secondfrequency range; and

(d) means for controlling the position of said shutter relative to saidedge of said strip comprising means, positioned on said second shield,sensitive to light within said second frequency range.

patent.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original UNITED RALPHG. NILSON, Primary Examiner.

M. ABRAMSON, Assistant Examiner.

